Temperature is controlled by means of heaters or air conditioners. To control the emperature of a small component, such as a crystal, the component is put in an oven, which has a resistive heater and is fairly well insulated. As more current is passed hrough the heater, it produces more heat; as less current is passed, the natural loss of heat through the insulated walls brings down the temperature. The temperature of a large room or building is controlled by means of a furnace or air conditioner, of course. Since hese usually require AC power at high currents and voltages, the microcomputer has to control a large AC current. An interesting problem in controlling air conditioners is due o the back pressure built up in them. If the air conditioner has just been running, is then turned off and is quickly turned on, the motor in it will stall because it cannot overcome the back pressure in it. So in controlling an air conditioner, if it is turned off, it must not be turned on for an interval of time that is long enough for the back pressure to dropoff.

Temperature is often sensed in a microprocessor system. Very high temperatures are measured indirectly, by measuring the infrared radiation they emit. Temperatures in the range -250°C to +1000°C can be measured by a thermocouple, which is a pair of dissimilar metals (iron and constantan, for instance), where the voltage developed between the metals is around 0.04 mV times the temperature. Note that such alow-level signal requires careful handling and amplification before it can be used in a microprocessor system. The most popular technique for measuring temperatures around room temperature is to put a constant current through a diode (or the diode in the emitter unction of a bipolar transistor) and measure the voltage across it. The output voltage is typically 2.2 mV times the temperature in degrees kelvin (°K). This voltage level requires some amplification before conversion to digital values is possible. Provided the current through the diode is held constant (by a constant current source), the transducer is accurate to within 0.1°K. While a common diode or transistor can be used, a numberof integrated circuits have been developed that combine a transistor and constant current source and amplifier. One of these (AD590) has just two pins, and regulates the current hrough it to be 1 uA times the temperature in kelvin. Converting to and then transmitting a current has the following advantage: the voltage drops in wires whose sensor is a long distance from the microprocessor, or in switches that may have unknown resistance, and thus does not affect the current. The current is converted to a voltage simply by passing it through a resistor. Finally, temperature can be sensed by a emperature sensitive resistor called a thermistor. Thermistors are quite nonlinear and have poor frequency responses, but relatively large changes in resistance result from changes in temperature.

7.1.3 Other Transducers

Pressure can be produced as a by-product of an activity controlled by a microcomputer. For instance, if a microcomputer controls the position of a valve, it can also control the flow of liquid into a system, which changes the pressure in the system. Pressure is sometimes measured. Usually, variations in pressure produce changes in the position of a diaphragm, so the position of the diaphragm is measured. While this can be mplemented with separate components, a complete system using a Sensym chip in the LX1800 series of chips (formerly a National Semiconductor series) can measure absolute